The present invention relates to an electric power converter for DC power transmission and frequency conversion and particularly relates to a high-voltage large-capacity electric power converter having a structure in which a plurality of self quenching type switching elements per one arm are connected in series.
With respect to gate electric sources adapted to switching elements for driving a high-voltage large-capacity electric power converter of the type having a structure in which a plurality of semiconductor switching elements are connected in series, there are a method in which insulation transformers are series-connected from the ground side so that respective gate drive circuits are supplied with electric power from the secondary side of the transformers as described in JP-Y 2-31909, and a method in which a gate power source is obtained from the potential of a main circuit in an apparatus using a thyristor as described in JP-B 53-40860. As another method, there is a method in which a gate power source is obtained from a main circuit through a current transformer on the basis of a snubber circuit current as described in JP-A 55-32449 and JP-A 63-124777.
However, for example, in a high-voltage large-capacity electric power converter for DC power transmission and frequency conversion as used in an electric power system, for example, the value of DC voltage reaches 125 kV. In the case where a 4.5 kV-grade large-capacity gate turn-off thyristor (hereinafter referred to as "GTO") is used as a self quenching type semiconductor element, the number of series-connected GTOs approaches 100 per one arm. Accordingly, in the case where an electric source for each of gate drive circuits for driving the GTOs is to be obtained from the series-connected insulation transformers, the capacity of an insulation transformer located in the lowermost stage from the ground side becomes a multiple of a capacity corresponding to the number of switching elements or corresponding to the number of modules each constituted by a plurality of series-connected switching elements so that apparatus size becomes large compared with the capacity of an insulation transformer located in the uppermost stage. In a method in which insulation transformers are provided correspondingly to respective potential values, the transformers can be provided so as to be equal in capacity but insulation between the primary winding (ground side) and the secondary winding (apparatus side) cannot be made easily. Further, gate drive electric power for a thyristor is required only at the time of turning-on thereof while gate drive electric power for a GTO is required both at the time of turning-on and at the time of turning-off. As the GTO device capacity increases or as the carrier frequency of the apparatus increases, the drive electric power increases. For example, in the case of the above-mentioned high-voltage large-capacity electric power converter, if electric power required for the gate of a 4.5 kV-3 kA-grade GTO is 200 W in the condition of 100 series-connected GTOs per one arm, three-phase bridge connection and carrier frequency of 500 Hz, the number of GTOs becomes 600. Accordingly, gate drive electric power required for the apparatus as a whole becomes 120 kW as bulk power. Therefore, the method in which a gate electric source is obtained from a main circuit, as described in the prior art, is considered. However, in the case where a self quenching type semiconductor switching element such as a GTO is to be driven, an idea different from the idea required for the thyristor is required so that it is necessary to apply an inverse-bias voltage to the gate in order to prevent maloperation before a voltage is applied to the main circuit. As a method for obtaining a gate electric source under the consideration of these problems, nothing has been described specifically.